[41.03] MHD Turbulence: Consequences and Techniques to Study

Huirong Yan (Dept of Astronomy, UW-Madison)

Astrophysical fluids are magnetized with magnetohydrodynamic
(MHD) turbulence playing a key role for various
astrophysical processes. In my thesis I study different
astrophysical implications of MHD turbulence for propagation
and acceleration of cosmic rays, dynamics of dust as well as
the new ways of observational studies of magnetic fields.

Using recently obtained scaling laws for MHD modes, we
identified fast modes as the dominant agent for cosmic ray
scattering for most of the interstellar phases. This
conclusion was reached in spite of the damping of fast modes
that I took into account. In addition, we found that the
traditional picture of shock acceleration is incomplete, as
it ignores the effect of preexisting turbulence in the
surrounding gas. Our research revealed suppression of
streaming instability, which is an essential component of
first order Fermi acceleration in shocks, by the ambient MHD
turbulence. This suppression limits the energy of cosmic
rays that can be accelerated by supernovae and invalidates
many conclusions reached on cosmic ray confinement for
models of galaxies embedded in fully ionized plasma.

We found that dynamics of charged grains is dominated by MHD
turbulence in a most of the interstellar environments. We
introduced new mechanisms of grain acceleration and
calculated shattering and coagulation rates of grains, as
well as the rate at which grains can adsorb heavy ions,
allow segregation of different grains and their alignment.
The obtained insight into grain dynamics is essential for
understanding dust physics, chemistry and evolution.

Another direction I have been working on is the
observational studies of astrophysical magnetic fields. We
studied alignment of atoms in the presence of anisotropic
radiation and magnetic field. Atoms can be aligned in terms
of their angular momentum by anisotropic radiation which is
common in astrophysical environment. The alignment is
modified by magnetic fields that cause precession of atoms.
We have identified atomic alignment enables a new tool to
study astrophysical magnetic fields. Since it allows also
temporal variations of magnetic fields, atomic alignment
provides a cost effective way to study MHD turbulence at
different scales.